It is known that heterogeneous suspensions are characteristically composed of different phases in their formulation, more explicitly, and can be the result of the combination of liquids with solid particles, which can vary from millimeters to submicrons in size. These suspensions also contemplate the combination of different immiscible fluids, constituting emulsions in the system liquid phase, with the possibility of also including air bubbles in the system.
These heterogeneous suspensions are present in different technological areas, such as building materials as concretes, mortars, fibrocement, prefabricated components, etc., soils, foods, cosmetic and mining products, etc.
One major difficulty involved with materials of such nature is related to the assessment of the behavior of these materials in fluid state, even to those that will be transformed into solids at some other processing stage.
The techniques for assessing the behavior of fluids in shear stress, known as rheological characterization techniques, have as their basic principle the submission of these fluids to controlled stresses or deformations. Countless methods described in the literature are classified into four categories according to the flow measurement or shear stress measurement procedure, which are:
Free flow tests—the material flows due to its own weight, without any confining, or an object penetrates the material as the result of gravitational force;
Confined flow tests—the material flows due to its own weight or under the application of pressure by means of a restricted orifice;
Vibration tests—the material flows due to the application of vibration;
Shear tests under rotational flow the material is submitted to rotational shearing in a parallel plate or concentric cylinders system
Another way of classifying the test methods is related to the shearing range in which the materials being analyzed are submitted, given that in practice two major test classes can be pointed: single point and multipoint.
The single point category encompasses the most of traditional tests employed in the control of concretes consistency, as indicated in Table 1, besides the Vane test—Amziane et al., 2005.
Table 1 contains the main methods for rheological characterization of fresh concrete, based on the classification given by “Nist”—Koehler & Fowler Koehler & Fowler, 2003.
TABLE 1NISTType ofClassificationTestParametersmeasurementFree flowTruncated coneYield Strengthsingle pointFree flowModified truncatedYield Strength;single pointconeConfinedOrimet TestViscositysingle pointFlowV-Funnel TestConfinedAbilityViscositysingle pointFlowFillingVibrationPowers RemoldingViscositysingle pointTestRotationalRheometersYieldmultipointRheometerStrength;Viscosity
Generally speaking, these tests characterize the materials in a single strength or shear rate condition. In the free and confined flow tests, the strength applied is proportional to the density of the material, while in the vibration tests, the shear rate applied is defined by the frequency and amplitude of the mobile element. In turn, the Vane test is a test that quantifies the yield strength of compositions.
Results obtained by means of such methods do not provide, therefore, a complete rheological characterization of concretes, with the possibility of resulting in wrong interpretations about the behavior of concretes in fluid state, under different application conditions.
Illustrating this concept, the FIG. 1 presents a schematic representation of three distinct “Bingham” fluids. The schematic describes the strength vs. shear rate profiles and the figures below the respective viscosities calculated as the ratio between strength and shear rate.
Table 1 presents the main rheological characterization methods according to the “NIST” classification, identifying the fundamental rheological parameters to which the test is related. The aforementioned presents, still, a second form of classification of these methods, based on the quantity of shear rates assessed during the single point and multipoint tests.
As observed, the ‘A’ and ‘B’ systems present the same yield strength, but the strength and fluid viscosity levels of ‘B’ are higher than those of system ‘A’ at the remaining shear rates.
In FIG. 1 it is verified the ‘strength profiles’×‘shear rate’, where the circles in orange highlight the shear rates with the equivalent viscosities calculated as the strength/shear rate ratio.
Besides the intrinsic uncertainties of the single point characterization concept, the presented test methods do not isolate the fundamental rheological parameters, with exception of the Vane test yield strength test. Overall, these methods provide results that are influenced both by the viscosity as by the materials' yield strength, as demonstrated in FIG. 2 for determination of fluidity by means of the consistency test similar to the truncated cone.
Finite element simulation results confirm that the consistency values present an inversed relation with the two rheological constants, yield strength and viscosity.
As a matter of fact, the rheological assessment of reactive concentrated suspensions with high viscosity, which consistency changes throughout time due to irreversible microstructural changes, in other words, cement hydration, polymerization, coagulation, etc., is a challenge in the field of rheology—Meeten, 2000.
Therefore, the identification of rheological parameters associated to the behavior of fluids and suspensions must be made my means of multipoint techniques that assess the behavior of materials at different shear rates and strengths, as in rheometry tests—Pileggi et al., 2000.
Such multipoint tests consist of the rheological characterization of fluids and suspensions in different conditions of shear strength and rate, enabling thus the simultaneous identification of fundamental rheological parameters, in other words, yield strength, viscosity and rheological profile. These types of tests applied to concretes are predominantly based on rotational rheometry—Ferraris, 1999.
The aforementioned rheometers are pieces of equipment dedicated to the assessment of rheological properties of fluids and suspensions, and allow the study of viscosity and yield strength behaviors as a function of other variables, such as time, temperature, etc.
The various rheometers for suspensions that are commercially available are based on only two basic functioning principles—Stein, 1986, which are: (a) rheometer in which the torque that is proportional to the strength applied to the fluid is controlled, and the resulting shear is assessed; (b) rheometer in which the shearing applied to the material is controlled, and the strength necessary for such is registered. Therefore, it is verified that the torque rheometers are indicated to assessments in which the strength requirements control the flow of material, while the shearing equipment are the most indicated to assessments of the rheological behavior in various flow rates.
Overall, the commercially available precision rheometers are not adequate to materials with an extensive granulometry, such as concretes and mortars, due to the fact that these rheometers can only act in restricted torque ranges, being limited to systems comprised of particles smaller than 100 μm.
Additionally, the commonly employed test geometries: concentric cylinders, parallel plate, cone-plate, capillary, vane, etc., tend to not be adequate to the assessment of concentrated systems or with addition of microparticles, which is the case of concrete and mortars.
It is known that the first rheometer to be specifically developed to the rheological characterization of concretes dates back to the 1960's, where the Powers model—Power, 1968—was based on the concentric cylinders model for the application of shear forces to the material. In this conception, the previously mixed concrete is poured into a cylindrical container in which a rotational element, also cylindrical, is introduced at the center of the mass, and the forces necessary the move the central cylinder are registered.
Therefore, based on this architecture, new models have been developed, such as ‘Wallevik’ and ‘Gjorv’ “Con Tec BML viscometer”; ‘Cousso’ “Cemagref-IMG”; ‘Tattersall’ and ‘Bloomer’ “Two-Point rheometer”—Brower, 2001. Besides the aforementioned models, the technological evolution of rheometers resulted in pieces of equipment that use other concepts for sheared material, as the parallel plate system developed by Larrard et. al. “Btrheom” and the planetary system proposed by ‘Beauprè’ “IBB rheometer”—Ferraris, 1999.
The rheometer for mortars and concretes developed at Poli-USP allows the use of two concepts of shear application: concentration or planetary rotation.
The use of rheometers has gained space, and the paper published in 1998 by the National Institute of Standards and Technology—NIST—‘Ferraris and Larrard, 1998’ can be highlighted among several others. In this paper, following a bibliographic review about the rheological characterization of construction concrete, it is proposed the use of rheometers for the characterization of high-performance self-yielding concretes.
The importance of using rheometers in the rheological characterization of concretes has been recognized in such manner that in September 2001, NIST published a report related to an international cooperation work carried out between eight countries, called ‘Comparison of concrete rheometers’: International tests at LCPC—Nantes, France, 2000—Ferraris & Brower, 2001.
The main purpose of this project consists in comparing, for the first time, the operational performance of distinct rheometers ‘Con Tec BML viscometer’, ‘Cemagref-IMG’, ‘Two-Point rheometer’, ‘Btrheom e IBB rheometer’, given that, there were no standards and norms for the functioning of such pieces of equipment at the time.
Therefore, the five rheometers were transported from their countries of origin for the Laboratoire central des pants et chaussèes (LCPC) in France, being simultaneously used in the assessment of the behavior of several concrete compositions.
Among the observations contained in the report, the main one was the confirmation that, in spite of the absolute differences between the measured values, the rheometer assess and classify in a similar manner the rheological behavior of distinct concretes, as observed by the yield strength and viscosity in FIG. 3, which provides the conditions for a future establishment of correlation curves between the devices.
In spite of the advantages associated to rheometry, the large dimensions of rheometers of concretes do not allow the portability that is necessary for their use in technological control in construction works, being, therefore, indicated for the development of compositions in laboratories and concrete centers.
This technology gap has been overcome with the arrival of portable rheometers, which consist of simplified, solely dedicated to the execution of standardized control tests in construction works.
One additional advantage of the use of rheometers for the characterization of concretes comes from the fact that the identification of its parameters and rheological profiles (see schematic example in FIG. 1), in different shear requests, occurs in a simultaneous manner. Therefore, one only rheometry test provides subsidies for defining the adequation of compositions to the proposed application methods.
The use of rheometers has enabled a better comprehension of factors that affect the behavior of cementitious materials. FIG. 4 presents the example of one schematic based on the paper by ‘Banfil 2005’, on the impact of different contents of water, air, microsilica (ultrafine particles) and dispersant on the viscosity and the mortars yield strength.
As observed, the increase of water and dispersant, represented by the sense of arrows, result in the decrease of both rheological parameters. However, the increase in air content practically does not affect the yield strength, in spite of a large impact on viscosity. At last, the increase in microsilica content reduces the viscosity, but it increases the yield
The use of the rheometry technique allows a detailed comprehension of the impact that each of the components has on the behavior of concretes in fresh state. Therefore, this characterization tool is fundamental for the preparation of a formulation methodology based on microstructural/rheological concepts.
However, all the pieces of equipment describe present some limitations, such as the rheometers ‘Com Tee BML viscometer’, ‘Cemagref-IMG’, ‘Two-Point rheometer’, ‘Btrheom’, ‘IBB rheometer’, portable rheometer and ‘Icar’ rheometer dedicated to a single family of tests, being comprised of a single set of container and shear geometry.
Another drawback is due to the fact that the mobile rheometers are not rigid, resulting in inaccurate measurements.
Another drawback is due to the fact that the existing rheometers do not have a system for compensating segregation, and, therefore, they do not allow the execution of long-term tests.
Another drawback is due to the fact that commercially available rheometers are developed for the assessment of concretes with fluid consistency or mortars that are not plastic.
Another drawback is due to the fact that the existing rheometers do not have a system for compensating segregation, and, therefore, they do not allow the execution of long-term tests.
Another drawback is due to the fact that commercially available rheometers are developed for the assessment of concretes with fluid consistency or mortars that are not plastic.
Another drawback is due to the fact that the existing pieces of equipment do not act in concretes that require high levels of torque, as dry materials.
Another drawback is due to the fact that the commercially available pieces of equipment are not able to mix materials.
Another drawback is due to the fact that conventional rheometers do not allow the adaptation of devices for mechanical tests.